Multilayer constructions are becoming of greater interest to the hybrid circuit manufacturers as circuits become increasingly complex, and the need for lower cost, higher line density circuits grows. Improvements in multilayer technology are critical to serving these growing demands.
There are several drivers in the marketplace. For example, lower cost is critical, which means a move to pure silver, platinum/silver, or copper metallurgies, as opposed to traditional gold or palladium/silver. Processing costs are equally important to the circuit manufacturer, so that minimizing the number of print and fire steps reduces the final circuit cost.
Finer line spacing is also necessary in the drive for miniaturization. Higher conductivity conductors are especially useful as the line widths shrink. Fortunately, this need is consistent with the trend to silver or copper metallizations.
Higher reliability dielectric layers are needed as the number of conductor crossovers in a circuit increases, especially with pure silver or copper conductors. For screen printed applications, customers increasingly want to move away from three print and fire steps to two or one for the dielectric in order to save processing costs. This trend has led to the development of more reliable, crystallizing glass systems. The crystallizing glasses are much more stable after firing than standard filled lead-borosilicate type of systems because the crystallization prevents further glass softening and flow after additional firings. However, this same level of reliability and stability in the dielectric makes it more difficult to bond standard thick film conductors to the substrate. Aged adhesion is usually degraded when the conductors are fired over previously fired dielectric. This is because the glasses and fluxes in the conductor do not react as well with the crystallized glass beneath it, owing to the stability of the crystallized dielectric.